The production of steel is historically a batch process comprising an oxidizing (or basic oxygen process) stage and a refining, or reducing stage. Generally, after the basic oxygen process, the coproduced slag floating on top of the molten steel will have a high iron oxide content and a high manganese oxide content. This slag follows the molten steel when it is poured into ladles and other vessels. The slag must be treated to reduce the iron oxide content.
If the iron oxide content of the slag is not reduced in the process of making aluminum deoxidized steel, the alumina that forms can cause caster nozzle clogging and surface defects. On the other hand, if the slag is excessively deoxidized, such that the resulting sum of iron and manganese oxides is less than 2%, then complex magnesium aluminate spinel inclusions may form. These can also cause nozzle clogging and quality defects. Furthermore, if too much slag reducing agent is added, the steel chemistry may be changed and miss the product specifications. The operators must also be aware if there is not enough free volume, or "freeboard" above the slag to accommodate the additional volume of slag treating agents to be added.
Although the concentration of iron oxide in the slag may be determined reasonably accurately, it has been difficult to determine the quantity of slag present, and many approaches have been followed to measure the thickness of the slag in order to decide how much aluminum, calcium carbide, or silicon to add to reduce the iron oxide. For example, a slag depth approximation may be computed from overall weight and the top slag level, since slag density can be determined reasonably accurately.
See Richard E. Kracich and Kenneth Goodson, "Ladle Slag Depth Measurement", 1996 Steelmaking Conference Proceedings, 53-60. In this paper, the authors describe a slag depth measuring system including measuring the lower level of slag by a "slag/steel interface electronics" coil, a probe which must penetrate the slag layer and beyond, where the induction effects of the coil are changed by the presence of the molten steel. Other kinds of probes used in the past include simple steel bars; the bar is inserted until its extremity is melted by the molten steel, and the thickness of the slag is assumed to be the length of the red-hot portion after the bar is pulled out. But frequently the upper crust of the slag does not heat the bar enough. Any device which must penetrate the slag to measure its depth must be expected not to last very long. Moreover, slag depth varies over its area, and the single probe techniques do not account for such variations. Nevertheless, such devices can often be relied upon at least for determination of the upper level of the molten steel, which is relatively constant and can be measured reliably at a single point.
Microwave radar has been used for years to measure the levels of various substances. The basic principle is known that the distance from the radar sender/receiver to the object under scrutiny is directly related to the time expiring between sending and receiving the radar signal, and the distance is accordingly readily computed from the elapsed time. In recent years, smaller and less expensive radar systems have become available, and their accuracy has been improved so that relatively close objects and levels of materials can be measured with confidence. The art has learned to measure or compensate for turbulence as well as unevenness in surfaces. Radar microwave is excellent for measuring a level below a layer of foam, which is otherwise hidden from view. Radar can penetrate the covers of some vessels, such as plastic lids, to determine the level of materials beneath them. The ability of contemporary computers to handle large quantities of data has also expanded the potential for radar usage.
Tezuka and Nagamune, in a paper entitled "M-Sequence Modulated Microwave level Meter and its Application" presented at the 1994 Steelmaking Conference Proceedings, 181-185 describe the measurement of the level of molten iron in a moving car, using a microwave technique; they also measure the top level of slag in a vessel. However, the microwave emissions were not shaped and accordingly a large portion of radiation received represented reflections from the sides of the vessels.
We are interested in determining not merely the upper level of the slag, but the lower level as well, so we can calculate the quantity of slag to be treated with reducing and other materials, such as aluminum or calcium carbide. It may be noted that the above mentioned Kracich and Goodson paper contains the following statement: "A microwave unit for measuring slag depth was tested, but was not feasible due to time constraints, cost and durability in the harsh environment." Our technique has overcome these problems and many others we encountered.